1. Field of the Invention
The invention relates to a wheel bearing device.
2. Description of Related Art
Usually, a drive shaft for transmitting torque from a power source is coupled to a bearing device that supports a drive wheel of a vehicle, such as an automobile. FIG. 4 is a sectional view that shows an example of a conventional wheel bearing device for supporting a drive wheel. A wheel bearing device 100 includes a cylindrical inner shaft 101, an outer ring 103, and an inner ring member 104. A flange 101a, to which a wheel is fitted, is formed at one end portion of the inner shaft 101. The outer ring 103 is arranged radially outward of the inner shaft 101 via double-row tapered rollers 102. The inner ring member 104 is fitted to the outer periphery of the other end portion of the inner shaft 101. A shaft portion 106 (drive shaft) is inserted into the inner shaft 101 to be arranged radially inward of the inner shaft 101. The shaft portion 106 is formed to protrude from the distal end of a constant velocity joint 105. A plurality of spline teeth 108 is formed on the outer periphery of the shaft portion 106. The spline teeth 108 are spline-fitted to spline grooves 107 formed in the inner periphery of the inner shaft 101. In addition, a nut 109 is screwed to one end portion of the shaft portion 106. The inner shaft 101 is fastened between the nut 109 and the constant velocity joint 105. In this way, the shaft portion 106 and the inner shaft 101 are coupled to each other so as to be rotatable together with each other (see, for example, Japanese Patent Application Publication No. 2006-64146 (JP 2006-64146 A)).
In the wheel bearing device 100, the inner ring member 104 is fitted to the outer periphery of the other end portion of the inner shaft 101, so the other end portion of the inner shaft 101 are deformed such that the diameter thereof is reduced. Accordingly, the spline grooves 107 formed in the inner periphery of the inner shaft 101 are also deformed such that the diameter of the inner periphery of the inner shaft 101 is reduced. This may hinder insertion of the shaft portion 106 into the inner shaft 101.
If the axial length, or the like, of the inner ring member 104 is adjusted, it is possible to reduce deformation of the other end portion of the inner shaft 101. However, adjusting the axial length of the inner ring member 104 requires a change in the size, or the like, of each tapered roller. This may lead to a decrease in the performance of the bearing device. Therefore, from a viewpoint of maintaining the original performance of the wheel bearing device, adjusting the axial length, or the like, of the inner ring member 104 is not preferable.
Therefore, the spline grooves 107 are formed in the inner periphery of the inner shaft 101 as follows. First, the inner periphery of the inner shaft 101, to which the inner ring member 104 has not been fitted, is subjected to broaching. In this way, the spline grooves 107 having predetermined dimensions are formed. After that, the wheel bearing device 100 is assembled, and the inner ring member 104 is fitted to the outer periphery of the other end portion of the inner shaft 101. Then, the inner periphery of the inner shaft 101 is subjected to broaching again, and then subjected to finishing such that the spline grooves 107 have predetermined dimensions. Therefore, even if the inner shaft 101 is deformed such that the diameter thereof is reduced through the fitting of the inner ring member 104 to the inner shaft 101, the dimensions of the spline grooves 107 of the inner shaft 101 are adjusted into the predetermined dimensions through the finishing. In this way, it is possible to prevent occurrence of a situation where the shaft portion 106 cannot be spline-fitted to the inner shaft 101.
However, in the above-described wheel bearing device, the inner shaft 101 is subjected to broaching twice. Therefore, a large number of man-hours are required to manufacture the wheel bearing device. On the other hand, if broaching on the inner shaft 101 alone is omitted and spline grooves 107 are formed in the inner periphery of the inner shaft 101 only by performing broaching on the inner shaft 101 to which the inner ring member 104 has been fitted, it is possible to reduce man-hours for broaching. However, in this case, after the wheel bearing device 100 is assembled, spline grooves 107 are formed by machining once, so a large amount of cutting chips are produced. The produced cutting chips may adhere to the tapered rollers and raceways of the bearing device and foreign matter may enter the wheel bearing device 100. Therefore, it is not preferable to form spline grooves 107 by performing broaching only once after the wheel bearing device 100 is assembled and the inner ring member 104 is fitted to the outer periphery of the inner shaft 101. Rather than forming the spline grooves 107 in this way, it is more preferable to finish forming the spline grooves 107 by performing broaching on the inner shaft 101 before assembly of the wheel bearing device 100.